Thermoelectric materials (thermoelectric conversion material) belong to materials that convert thermal energy to electrical energy. The thermoelectric materials are roughly categorized as two types, i.e., n-type and p-type materials. Such n-type and p-type thermoelectric materials, when alternately connected in series electrically and arranged in parallel thermally, give a thermoelectric conversion element. The thermoelectric conversion element, when receiving a temperature difference between both sides thereof, can generate electricity. The thermoelectric conversion element, when receiving a voltage between both terminals thereof, generates a temperature difference.
Exemplary common thermoelectric materials include Bi—Te intermetallic compounds. The compounds are widely used because of having a high Seebeck coefficient, namely, having relatively good electric generation efficiency (Non Patent Literature (NPL) 1). Exemplary general thermoelectric materials further include Pb—Te intermetallic compounds and Zn—Sb intermetallic compounds; and thermoelectric materials other than intermetallic compounds, such as oxide thermoelectric materials (NPL 1).
The present inventors have proposed a thermoelectric material in Patent Literature (PTL) 1. This thermoelectric material has a Heusler alloy type crystal structure and has a structure corresponding to a basic structure having a total number of valence electrons of 24 per chemical formula. The thermoelectric material has a total number of valence electrons per chemical formula controlled by controlling its chemical compositional ratio without substituting another element for part of a constitutional element, or by substituting another element for part of a constitutional element. An exemplary basic structure listed in the literature is Fe2VAl. An exemplary thermoelectric material whose compositional ratio is controlled without substituting for part of a constitutional element is one expressed by (Fe2/3V1/3)3−nAl1+n, where n is −0.048 to 0.052. This compound has a total number of valence electrons of 23.79 to 24.19 (Experimental Example 1 in PTL 1).
In addition, the present inventors have proposed another thermoelectric material in PTL 2. This thermoelectric material has a structure based on an Fe2VAl basic structure, except for substituting another element for part of a constitutional element and adjusting, for example, an atomic weight of the other element to substitute.
The Fe2VAl-based thermoelectric materials according to these proposals act as n-type materials when having a total number of valence electrons of 24 or more; and act as p-type materials when having a total number of valence electrons of 24 or less. Typically, an n-type Fe2V(Al1−αMα), where M is Si, Ge, or Sn, and 0<α<1, having a total number of valence electrons of 24 or more has a large Seebeck coefficient of about −120 μV/K (PTL 1, PTL 2, and NPL 2). In contrast, a p-type Fe2(V1−αMα)Al, where M is Ti and 0<α<1, having a total number of valence electrons of 24 or less has a Seebeck coefficient of about +80 μV/K (PTL 1 and NPL 3).
Of thermoelectric materials, those having better electric generation efficiency have been demanded. To meet this demand, the present inventors have proposed a thermoelectric material having a structure based on an Fe2VAl basic structure, except for substituting other elements for at least part of Fe and V in PTL 3.
In the thermoelectric material just mentioned above, an element M1, when defined as another element substituting for Fe, is selected from the group consisting of elements of Groups 7 to 10 in Periods 4 to 6 in the Periodic Table; and an element M2, when defined as another element substituting for V, is selected from the group consisting of elements of Groups 4 to 6 in Periods 4 to 6 in the Periodic Table. The thermoelectric material has amounts α and β of substituting elements M1 and M2, which amounts satisfy General Formula (Fe1−αM1α)2(V1−βM2β)Al and are controlled within ranges of 0<α<1 and 0<β<1. The thermoelectric material is controlled to act as a p-type material by having a total number of valence electrons of less than 24 per chemical formula; or is controlled to act as an n-type material by having a total number of valence electrons of more than 24 per chemical formula. Typically, a thermoelectric material including Ir as element M1 and Ti as element M2 has a Seebeck coefficient of about +90 μV/K and has a higher power factor and a higher figure of merit than those of a thermoelectric material substituting only Ti as M2.